Process of obtaining constituents of air having a higher boiling point than oxygen



June 19, 1934. P, SCHUFTAN 1,963,309

PROCESS OF OBTAINING CONSTITUENTS OF AIR HAVING A HIGHER BOILING POINTTHAN OXYGEN Filed Sept. 24, 1932 0: IV: fl: +1; 1 5 4 man I):

Patented June 19, 1934 PROCESS OF OBTAINING CONSTITUENTS OF AIR HAVING AHIGHER BOILING POINT THAN OXYGEN Paul Schuftan, Hollriegelskreuth, nearMunich, Germany, assignor to Gesellschaft fiir Lindes Eismaschinen A.G.,

Munich, Germany Hollriegelskreuth, near Application September 24, 1932,Serial No. 634,779 In Germany November 14, 1931 3 Claims.

The present invention relates to a process for obtaining constituents ofair having a higher boiling point than oxygen; for example, krypton andxenon. For the sake of brevity, krypton will 5 be used in the followingspecification and claims as representative of such higher boilingconstituents.

It is known that in the separation of the constituents of air by coolingto low temperatures 0 only a small portion of the krypton is found inthe liquid oxygen remaining in the evaporator. The greater portion ofthe krypton is contained in the gaseous oxygen withdrawn, and in fact inabout five times the concentration in which it is found in atmosphericair. It has previously been proposed to obtain the krypton from thegaseous oxygen by selective absorption in carbon or silica gel, but thismethod has serious disadvantages, of which only the discontinuity of theprocess, the

relatively small adsorptive capacity of the adsorbents for krypton atits low partial pressure, and the diificult removal and working up ofthe absorbates will be mentioned. It has also been proposed to obtainthe krypton from the liquid portion of the oxygen which remains in theevaporator of the separation apparatus. For the reasons mentioned above,a quantitative recovery of krypton cannot be obtained in this manner.Moreover, difiiculties are met in the rectification of the liquidbecause of the rapid obstruction of the plates of the column with thesolid impurities, such as ice and'solid carbon dioxide contained in theliquid oxygen.

All of these disadvantages are avoided by the present invention. It hasbeen found that the krypton can be obtained in very high concentrationand yield by washing it out of the gaseous oxygen by means ofkrypton-free oxygen in countercurrent. This result was all the more sur-40 prising in that it appeared quite improbable that a constituentcontained in a gas in the extremely low concentration of 5 parts permillion could be obtained in good yields by a technical washing process.

Nevertheless, the washing out of krypton can be effected verysatisfactorily when carried out in accordance with the invention,particularly if an efficient rectification column is used. A moresatisfactory and uniform method of operation is obtained by using thegaseous oxygen, because this no longer contains any impurities which canseperate on the rectifier plates in solid form.

The use of a krypton-free or practically krypton-free washing liquid isessential for the successful operation of the process. The liquidnitrogen obtained in the separation of air could be used forthis'purpose, because it has the greatest difference in boiling pointfrom krypton of any of the constituents of air. However, since itmingles with the separated oxygen in the washing process and thereforeseriously contaminates it, krypton-free oxygen is used as the washingliquid, which is obtained by liquefying a portion of the separatedoxygen by deep cooling in the upper part of the washer.

The counter-current washing of the krypton out of the gaseous oxygen iseffected most simply in a vertical tube sheaf, into the lower part ofwhich the gaseous oxygen from the evaporator of the separation apparatusflows at about its boiling temperature. The tube sheaf is suitablycooled externally, advantageously by the cold nitrogen coming from theapparatus, which in temperature and amount suffices for the formation ofthe necessary amount of washing liquid. The formation of the washingliquid can also be effected in a condenser located at the head of thewashing column and supplied with liquid air or liquid nitrogen.

It is particularly advantageous to use an efficient rectifying columnfor the washing apparatus, into the lower part of which the oxygen vaporfrom the evaporator is supplied, while a condenser is provided at thetop of the column.

The further enrichment of the washing liquid in krypton can be effectedmost simply by returning the washing liquid from the washer into theoxygen evaporator of the air separation apparatus. In this way thekrypton becomes more and more concentrated in the liquid oxygen in theevaporator, which at the end of a period of operation may be drawn 01fand worked up into pure krypton.

Another method of enrichment which affords the possibility ofcontinuously producing krypton of any desired concentration consists inpassing the liquid oxygen coming from the washing column into arectification column, advantageously situated under the washing column,and heated at its lower end. The vapors coming from this auxiliarycolumn are returned to the lower part of the washingcolumn. In thismethod of operation, in order to obtain a quantitative yield of krypton,it is advantageous at' the end of a period of operation of theseparation system to vaporize the liquid oxygen remaining in theevaporator into the washing column, or a small amount of liquid oxygenin the evaporator may be allowed to continuously flow into the auxiliarycolumn, in order to avoid the concentrating of krypton in the condenser.

The invention will be moreparticularly described for. the purpose ofillustration'with reference to the accompanying drawing in which Fig.

l is a diagrammatic representation of apparatus for carrying out oneembodiment of the invention, and Fig. 2 is a diagrammatic representationof apparatus for carrying out another em"- bodiment of the invention.

In Figure 1, 1 is the high pressure column, 2-

the upper column, and 3 the evaporator and condenser of an airseparating apparatus. The krypton-containing gaseous oxygen coming fromthe evaporator at 4 passes into-the vertical tube sheaf 5. Gaseousnitrogen from the separator flows through the jacket 6 and passes out at8.

Since the nitrogen is-about 13 colder than the oxygen, and in amount isabout 4 times the amount of the oxygen, a copious condensation of oxygentakes place in the upper part of 5. The downwardly'fiowing liquid'washesthe krypton out of the upwardly flowing gaseous oxygen and is therebylargely revaporized and thus enriched takes place. At the end'of theperiod of operation of the separation system the liquid in theevaporator, containing substantially all of the krypton of the treatedair, is drawn 01f at 9 and worked up into pure krypton in known manner.

In the method illustrated in Figure 2 the washing out and concentrationof the krypton is efiected in rectification columns. Thekryptoncontaining gaseous oxygen from the evaporator 3 of the airseparation apparatus is led into the washing column 4, which is providedwith rectifying plates. The condensation of the kryptonfree washingliquid takes place in condenser 5 which is cooled with liquid nitrogensupplied through valve 8. The operation of column 4 corresponds inprinciple with the previously described operation of tube sheaf 5 ofFigure 1. The krypton-containing liquid oxygen at the bottom of thecolumn passes into the enriching column 6 and is there furtherconcentrated. The heating of this column isefiected by means ofcompressed air the amount of which is regulated by valve 9. The highlyconcentrated krypton can be drawn oil in gaseous form at 10 or can bedrawn off from the evaporator 7 in liquid form. By this method ofoperation it is possible to obtain a continuous production of krypton ofany desired concentration.

The operation of column 4 and condenser 5 will be explained in moredetail by an example:

It will be assumed that 100 cubic meters of krypton-containing gaseousoxygen pass from 3 into column 4 and that this gas is in equilibriumwith a liquid containing 10 times its concentration of krypton. If theliquid passingv from column 4 into column 6 is to contain all of thekrypton, its amount must be 10 cubic meters; that is, on the assumptionof the same heat of vaporization for krypton and oxygen, 10 cubic metersof oxygen must be condensed in condenser 5.

When the system is put into operation a krypton-containing liquid willat first condense in 5,

and in fact the first drops condensing will have a krypton content tentimes that of the gas, a

corresponding to the equilibrium; in the course of further condensation,however, because of the progressive impoverishment of the gas phase inkrypton, the condensate-becomes poorer and poorer in krypton. On thewhole, therefore, immediately after putting the system in operation thecondensate will contain, it is true, some krypton, but in much lowerconcentration than and likewise the condensate therefrom. The

impoverishment of the rising vapors in krypton continues until apermanent condition is finally reached wherein only krypton-free oxygenreaches the condenser, in which 10 cubic meters of krypton-free oxygenare condensed, while 10 cubic meters of a mixture of liquid oxygen withall of thekrypton in the entering vapors leave the bottom'of the column.

The operation of the apparatus of Figure 1 differs only from astructural point of view, the principle of operation being the same.

In a prior method krypton-containing oxygen was fed into the top of acolumn in liquid form.

The gasesdrawn off had to be in equilibrium with this liquid andtherefore must contain krypton, so that an'appreciable part of thekrypton must be lost with the vapors. On the contrary, by the process ofthe invention,'it is possible for the first time to obtain all of thekrypton contained in the air.

The concentration of krypton in the oxygen depends on the length of timethe air separation apparatus is operated. In the method of operationillustrated in Fig. 1 substantially all of the krypton present in theair'treated in a given period of operation will be found in theevaporator liquid. Since the amount of liquid in the evaporator isrelatively large and the amount of krypton in the air is only1:1,000,000, at the end of a normal period of operation only a smallpercentage of krypton will be found in the evaporator liquid. In themethod of operation illustrated in Fig. 2, it is possible to furtherconcentrate the krypton-containing oxygen by rectification in column 6,so that it is possible, after a sufliciently long period ofoperation,'to draw ofl pure krypton at 10.

A further advantage of the new method is that by utilizing gaseousoxygen no obstruction of the rectifier plates by impurities isencountered, since it contains no noticeable trace of harmfulimpurities.

The method of the invention provides a continuous and quantitativeseparation of krypton from the air in heretofore unattainedconcentration. It can be operated with the simplest means and at verylow cost. Together with the krypton, other substanceshaving higherboiling points than oxygen are obtained, particularly xenon.

I claim:

1. An improvement in the method for obtaining krypton and xenon andother constituents of air having a higher boiling point than oxygenwhich comprises separating air by low temperature cooling andrectification into a low'boiling portion consisting of constituents ofair of lower boilng point than oxygen and a high boiling portioncontaining substantially all of the oxygen, vaporizing oxygen from saidhigh boiling portion, and subjecting all of the oxygen vaporized fromsaid high boiling portion to countercurrent washing with liquid oxygenwhich has been freed of its krypton content by rectification.

2. An improvement in the method for obtaining krypton and xenon andother constituents of air having a higher boiling point than oxygenwhich krypton-containing condensate to said high boiling portion. 1

ing krypton and xenon and other constituents of air having a higherboiling point than oxygen which comprises separating air by lowtemperature cooling and rectification into a low boiling portionconsisting of constituents of air of lower boiling point than oxygen anda high boiling portion containing substantially all of the oxygen,vaporizing oxygen from said high'boiling portion, passing all of theoxygen vaporized from said high boiling portion into the lower part of arectification column cooled at its head with liquid nitrogen andsubjecting the krypton-containing liquid from said column torectification in a subjacent column from which the vapors pass into theupper column.

' PAUL SCHUFTAN.

